1. Field of the Invention
The present invention relates to a multilayer printed circuit board. More particularly, the invention relates to a multilayer printed circuit board having improved heat resistance and excellent electrical and mechanical properties, and a process for producing the same.
2. Description of Prior Arts
Previously, most of solid-state elements or parts mounted to a printed circuit board have been in the form of individual parts including active elements and passive elements. However, along the technological development, the tendency is clear that IC or LSI shall be more densely mounted to a printed circuit board. A printed circuit board is expected to have the capacity of more dense and intricate mountings.
However, the conventional process for producing a printed circuit board is not employable to produce a densely mounted circuit board, and a new process for enabling the production of a highly dense circuit in the form of a multilayer board is expected.
For example, in order to increase the number of circuit lines on a printed circuit board, a plating through hole process to produce multilayer structure has been widely used. However, in this process the circuit area space of the circuit pattern is restricted by the space of land portion. This means that the conventional process inherently possesses a defect in which the circuit area space is more reduced, as the number of layer of a printed circuit board is increased. This is because the number of through holes for connecting layers should be increased, as the number of layers are increased. Therefore, as far as the conventional process is concerned, it is almost impossible to produce, in view of both materials and precisions, ten or more layers while minimizing the line width and the size of the through hole. Accordingly, a new circuit-producing technology is required to give a printed circuit board having an increased number of circuit layers.
In order to meet the above-mentioned requirements, it can be thought that a process forming the via hole using the photofabrication technology. More particularly, this process comprises the following steps of: forming a circuit pattern of the first layer over an insulating substrate; forming a photo-polymer layer over the circuit pattern; exposing the photo-polymer layer to radiation to photoset the polymer layer except for a predetermined portion where at least one via hole is formed; developing the exposed layer to produce a photoset polymer film of the first layer having at least one via hole therein; forming a circuit parttern of the second layer over the photoset polymer film of the first layer; and repeating the above-described steps of forming a photo-polymer layer and producing a photoset polymer film having at least one via hole to prepare a multilyaer printed circuit pattern on the substrate.
To produce a multilayer printed circuit board by the above-described process, the photo-polymer is required to possess various specific properties appropriate as a fine photosensitive material and further to possess an appropriate property for forming the interlaminar insulating film, but the conventional photo-polymers hardly fullfill such required properties and are considered to be unsatisfactory for the production of the above-described multilayer structure.